As a parking-time discharge current which a battery discharges during parking, a vehicle dark current, a consumption current of a battery control device, a self discharge current and the like are named. A vehicle dark current is an electric current which flows into a certain electric load (specific electric load) of a vehicle from the battery during parking. As a certain electric load, a backup memory, security equipment or the like can be exemplified. A self discharge current is an electric current consumed by a reaction (for example, a reaction between an active material and an electrolyte solution or the like) in the battery, and is an electric current which is constantly consumed regardless of a use state of the battery.
Such a parking-time discharge current is usually an extremely small electric current of 100 mA or less and hence, when a current sensor is required to have a large current measurement range, there exists a drawback that a measurement error becomes large. To suppress such a measurement error of the parking-time discharge current, a method is considered where a current detection part having high sensitivity is provided exclusively for detecting an extremely small electric current. However, the use of such a dedicated current detection part (current sensor) increases the number of parts so that a manufacturing cost is pushed up. Japanese Patent Publication No. 5141424 (patent literature 1) discloses a technique where sensitivity (gain) of a current detection part is adjusted for increasing measurement accuracy of a vehicle dark current. Although the current detection part capable of adjusting sensitivity as described in patent literature 1 exhibits high performance, the current detection part is expensive so that a manufacturing cost is eventually pushed up.